1. Field of the Invention
The present invention relates to a switching power supply device that includes a switching element on a primary side and a rectifier circuit on a secondary side and that performs power transmission by using the resonance phenomenon.
2. Description of the Related Art
With recent progress toward compact and lightweight electronic devices, market demand for high-efficiency, compact, and lightweight switching power supplies has increasingly grown. For example, current resonant half-bridge converters that cause a transformer to operate in response to a sinusoidal resonant current flowing therethrough by using a LC resonance phenomenon have been increasingly put into practical use in markets of goods such as flat-screen televisions with comparatively moderated output current ripple characteristics, while taking advantage of the feature of high efficiency.
For example, a LC series resonant DC-DC converter is disclosed in Japanese Unexamined Patent Application Publication No. 9-308243. FIG. 1 is a basic circuit diagram of a switching power supply device in Japanese Unexamined Patent Application Publication No. 9-308243. The disclosed switching power supply device is a current resonant type half-bridge DC/DC converter, and a LC resonant circuit composed of an inductor Lr and a capacitor Cr and two switching elements Q1 and Q2 are connected to a primary winding np of a transformer T. A rectifying and smoothing circuit composed of diodes Ds1 and Ds2 and a capacitor Co is provided at secondary windings ns1 and ns2 of the transformer T.
With the configuration described above, the switching elements Q1 and Q2 are alternately turned on and off with a dead time, and the current waveform flowing through the transformer T is a sinusoidal resonant waveform. Further, power is transmitted from the primary side to the secondary side during both on periods and off periods of the two switching elements Q1 and Q2.
An existing current resonant type converter requires its switching frequency to be changed to control its output power. The switching power supply device disclosed in Japanese Unexamined Patent Application Publication No. 9-308243 is also configured to perform frequency control and PFM (Pulse Frequency Modulation) control for control of its output.
For this reason, it is necessary to design a smoothing circuit in accordance with the minimum operating frequency, which hinders compactness. Further, variation of the switching frequency increases difficulty in controllability for an increase in frequency. In addition, design for high-efficiency operation at a rated load (high power) may reduce efficiency at a low load.
Additionally, there are many challenges in performing operation at a high frequency on the order of MHz for miniaturization of magnetic components. That is, in order to address variations in a certain input voltage or variations in output power, if the variation range of the switching frequency is spread in accordance with the variations, in terms of EMC (“electromagnetic compatibility”, electromagnetic non-interference and tolerance of electric devices and the like), variation of the switching frequency increases the frequency range of noise to be suppressed or the frequency range for which the tolerance is to be enhanced, and makes it difficult to take EMC measures.